The continuous culturing of microbes (i.e., microorganisms) is, in principle, well known for the production of large quantities of microbes, both as a research tool and for the preparation or isolation of microbial products. Thus, such continuous culturing is useful for general research purposes, such as the isolation of such cellular components as enzymes, cell walls, cytochromes, and the like; as a source of microorganisms for industrial biochemical processes, such as the production of fermented beverages, antibiotics, alcohol, organic carboxylic acids, cheeses, and the like; for the production of single-cell protein as an animal feed supplement; and for the production of bacterial vaccines, to name but a few examples.
In the past, the continuous culturing of microbes most often has utilized either the chemostat or the turbidostat. See, e.g., J. R. Norris and D. W. Ribbons, Editors, "Methods in Microbiology", Vol. 2, Academic Press Inc. (London) Ltd., London, 1970, pp. 259-276 and 349-376.
In the chemostat, a culture of fixed volume is contained in a suitably-constructed vessel to which medium is pumped at a constant rate. Typically, constant culture volume is achieved by employing a vessel which has a constant level overflow device. The medium is compounded to contain an excess of all substances essential for maximum microbial growth, except for one which is present in a growth-limiting amount. As a consequence, the culture can grow only at a rate which is directly proportional to the medium flow rate.
The turbidostat is a continuous-culture apparatus in which a photoelectric monitor detects deviations from some pre-selected culture turbidity (i.e., constant microbial population density) and passes a signal calling for a compensatory increase or decrease in dilution rate to a pump or valve controlling the flow of growth medium. Apart from the photoelectric control system and some simple optical requirements with respect to the culture vessel, all of the components of a turbidostat have functions similar to those of a chemostat.
In practice, both the chemostat and the turbidostat are designed to utilize microbial specific growth rates of less than about one-half the maximum specific growth rate. Furthermore, both devices are subject to wash-out which occurs when the rate of dilution (i.e., medium flow rate) is increased beyond the culture growth rate. Because there is a significant lag in the adjustment of growth rate to dilution, such lag and the problems associated with wash-out must be taken into account in continuous culture systems. See, e.g., R. I. Mateless et al., Nature, 208, 263 (1965).